MANAGUA, Nicaragua (Reuters) - Nicaragua's former Marxist guerrilla leader Daniel Ortega bounced back to power on Tuesday in a presidential election victory that bolsters an increasingly assertive anti-U.S. bloc in Latin America.

Ortega won with 38 percent of the vote, 9 points ahead of his Washington-backed conservative rival Eduardo Montealegre.

Ortega, who seized power in a popular 1979 revolution and fought U.S.-backed Contra rebels as president in the 1980s, was conciliatory in victory but the White House warned its support for Nicaragua would hinge on his commitment to democracy.

The 60-year-old president-elect met Montealegre late on Tuesday and both promised to work together to attack poverty and encourage the private investment needed to create jobs.

"We thank God for this chance to build a Nicaragua in reconciliation by talking to each other and reaching consensus, even with our differences," Ortega said.

His victory in a third comeback attempt was a huge boost for Venezuelan President Hugo Chavez, who is trying to build a Latin American alliance of anti-U.S. leaders.

The two friends spoke by telephone on Tuesday night and Ortega said he was sure Chavez would be re-elected next month.

"I'm convinced that, as we have won today, our Venezuelan brother will have a new victory on December 3 and continue the struggle for justice, peace and solidarity between peoples," he said in a chat broadcast on Venezuelan state television.

"Latin America is leaving forever its role as the backyard of the North American empire. Yankee go home! Gringo go home! This land is ours, this is our America!" a delighted Chavez had said.

Thousands of left-wing Sandinista supporters took to the streets to celebrate Ortega's triumph, setting off fireworks and waving black-and-red party flags.

The poorest country in the Americas after Haiti, Nicaragua has never recovered from the civil war that killed 30,000 people and ruined its economy. Three pro-Washington governments that have ruled since Ortega's 1990 election defeat did little to alleviate poverty and were hit by corruption scandals.

U.S. CAUTIOUS

The White House made clear Ortega would have to earn continued U.S. support for Nicaragua.

"We will work with their leaders based on their commitment to and actions in support of Nicaragua's democratic future," said Gordon Johndroe, spokesman for the White House National Security Council.

Venezuela backed Ortega's campaign by sending cheap fertilizer and fuel to Sandinista-led groups. It is widely expected to now finance social programs in Nicaragua and some Ortega followers hope Chavez, rich with petrodollars, will help Nicaragua stand up to Washington.

"Whatever Chavez sends us helps us a lot and it makes us less scared because we know we are not alone, we have his support," said Miguel Mendoza, 45, who was orphaned at age 9 when his parents were killed by troops fighting against the Sandinista revolutionaries.

Ortega has dropped his Cold War-era Marxism and now speaks mainly of God, peace and reconciliation. He also backs a trade deal with the United States, but U.S. officials still do not trust him.

Washington recently warned of a cut in investment and aid if Ortega was returned to power, and some senior officials in President George W. Bush's administration have a long history of opposition to the president-elect.

They include Elliott Abrams, who serves on the National Security Council and was a key figure in the Iran-Contra affair in the 1980s, when the U.S. government secretly sold arms to Iran to channel funds to the Nicaraguan Contra rebels.

Ortega knows the cost of confronting Washington. The Contra war and a U.S. economic embargo in the 1980s wrecked the Sandinistas' ambitious education and health programs.

Combined with his Marxist government's mismanagement and heavy-handed repression of dissent, the U.S. pressure finally toppled Ortega when voters turned against him in 1990.

Ortega was helped back to power by divisions in the right, which had in previous elections united behind a single candidate to keep him out.

Still, he is despised by many who blame him for the bloodshed and chaos. Even some who voted for him are wary.

"I hope I didn't make a mistake. I gave him my vote of faith. He has to govern and meet all his promises," said Cecilia Rivas, a 25-year-old student.

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